The dominant commercial process for the manufacture of adipic acid involves the air-oxidation of cyclohexane to form a mixture of cyclohexanol and cyclohexanone, which is subsequently oxidized with nitric acid to form a mixture of carboxylic acids, including adipic acid.
Another patented process (Burke U.S. Pat. No. 4,788,333 and U.S. Pat. No. 4,788,334) for making adipic acid involves the hydrocarboxylation of pentenoic acids and their esters. Pentenoic acids and their esters, and in particular 3-pentenoic acid, are available from butadiene and butadiene derived feedstocks by metal catalyzed hydrocarboxylation. Such processes have the potential of providing high yield, high rate, low pollution processes for the manufacture of adipic acid.
In the hydrocarboxylation of 3-pentenoic acid and its esters, undesirably high levels of branched products are formed. Burke (U.S. Pat. No. 4,788,333) has disclosed that, for the production of adipic acid by the hydrocarboxylation of pentenoic acids, high linear selectivity is obtained when an iodide promoted rhodium catalyst is employed in conjunction with selected halocarbon solvents. While better linear selectivity is obtained in halocarbon solvents, their use results in additional costs associated with the containment, recovery and recycle of these volatile, environmentally hazardous solvents.
Burke discloses in U.S. Pat. No. 4,939,298, that branched diacids can be isomerized to adipic acid by heating in the presence of carbon monoxide and an iodide or bromide promoted rhodium catalyst.
The most commonly used promoters for metal catalyzed hydrocarboxylation also promote the conversion of pentenoic acids and esters to valerolactones. These lactones normally consist primarily of gamma-valerolactone (hereinafter referred to as valerolactone). Valerolactone can be hydrocarboxylated to adipic acid as described in Burke EPO published application 0395038, but there are costs and yield losses associated with such processes.
In U.S. Pat. No. 4,788,334 Burke discloses that the rate of hydrocarboxylation of pentenoic esters in selected aromatic and halocarbon solvents is accelerated by the addition of an aliphatic or aromatic acid having a pKa in the range of 4.2 to 5.2.
Craddock et al. (U.S. Pat. No. 3,816,489) have disclosed a process for the production of terminal carboxylic acids from ethylenically unsaturated compounds by hydrocarboxylation "in the presence of catalyst compositions essentially comprising iridium compounds and complexes, together with an iodide promoter in critical proportions". U.S. Pat. No. 3,816,488 discloses that similar results can be obtained using rhodium catalysts. The preferred reaction solvents are monocarboxylic acids having 2 to 20 carbon atoms.
The process of this invention seeks to overcome the disadvantages of the prior art by providing a high yield, high rate, low-pollution process for the manufacture of adipic acid from pentenoic acids and esters. The process of this invention gives high linear selectivity while avoiding the use of halocarbon solvents and results in the formation of relatively low levels of valerolactone.